Hi JensKuhne,
That does sound a bit puzzly, especially when this customer has been receiving invoices from you without any trouble before. A rejected email can happen when the recipient’s email address has ended up on a block list. For this one, it’d be best to reach out to our support team so they can check whether that address needs to be removed from the block list. The quickest path is to contact them through Live Chat via our virtual assistant, MOCA, so they can take a closer look from there.
Cheers,
Doreen